Added failing test for #1343.

tests/Avalonia.Visuals.UnitTests/Media/RectangleGeometryTests.cs

@@ -0,0 +1,39 @@
+using System;
+using Avalonia.Media;
+using Avalonia.Platform;
+using Avalonia.UnitTests;
+using Moq;
+using Xunit;
+
+namespace Avalonia.Visuals.UnitTests.Media
+{
+    public class RectangleGeometryTests
+    {
+        [Fact]
+        public void Rectangle_With_Transform_Can_Be_Changed()
+        {
+            using (UnitTestApplication.Start(GetServices()))
+            {
+                var target = new RectangleGeometry
+                {
+                    Rect = new Rect(0, 0, 100, 100),
+                    Transform = new RotateTransform(45),
+                };
+
+                target.Rect = new Rect(50, 50, 150, 150);
+            }
+        }
+
+        private TestServices GetServices()
+        {
+            var context = Mock.Of<IStreamGeometryContextImpl>();
+            var transformedGeometry = new Mock<IGeometryImpl>();
+            var streamGeometry = Mock.Of<IStreamGeometryImpl>(x => 
+                x.Open() == context &&
+                x.WithTransform(It.IsAny<Matrix>()) == transformedGeometry.Object);
+            var renderInterface = Mock.Of<IPlatformRenderInterface>(x =>
+                x.CreateStreamGeometry() == streamGeometry);
+            return new TestServices(renderInterface: renderInterface);
+        }
+    }
+}